Walking doll having legs with inwardly directed sections from which projections extend forwardly into a torso groove for pivotal mounting of the legs

ABSTRACT

A hollow doll&#39;&#39;s torso has hip openings which receive inturned portions atop the doll&#39;&#39;s legs. These portions mount the legs on the torso and have various radial protrusions. One protrusion, forward of the center of gravity of each leg when the leg is vertical, engages a recess in the doll body to form a hinge about which the leg swings forwardly when the torso is erect. The remaining protrusions serve as motion stops and guides in conjunction with formations within the doll&#39;&#39;s torso. As the torso is rocked side-to-side and advanced, opposite legs alternately pivot forward to simulate walking.

United States Patent Robert K. Ostrander 497 Prospect SL, Maplewood, NJ. 07040 776,502

Nov. 18, I968 July 13, 1971 Inventor Appl. No. Filed Patented WALKING DOLL HAVING LEGS WITH INWARDLY DIRECTED SECTIONS FROM WHICH PROJECTIONS EXTEND FORWARDLY INTO A TORSO GROOVE FOR PIVOTAL MOUNTING OF THE LEGS Primary Examinen-F. Barry Shay Attorney-Kirschstein, Kirschstein, Ottinger and Frank ABSTRACT: A hollow dolls torso has hip openings which receive inturned portions atop the dolls legs. These portions mount the legs on the torso and have various radial protrusions. One protrusion, forward of the center of gravity of each leg when the leg is vertical, engages a recess in the doll body to form a hinge about which the leg swings forwardly when the torso is erect. The remaining protrusions serve as motion stops and guides in conjunctionwith formations within the dolls torso. As the torso is rocked side-to-side and advanced, opposite legs alternately pivot forward to simulate walking.

PATENIEDJuusmn' 3,591,976

' SHEETIUFZ w INVENTOR ROBERT K OSTRANDER aw wi ATTORNEYS PAIENIED JUL 1 3 I971 SHEET 2 BF 2 ATTORNEY S WALKING DOLL HAVING LEGS WITH INWARDLY DIRECTED SECTIONS FROM WHICH PROJECTIONS EXTEND FORWARDLY INTO A TORSO GROOVE FOR IIVOTAL MOUNTING OF THE LEGS BACKGROUND OF THE INVENTION 1. Field of the Invention Dolls capable of simulating a walking motion.

2. Description of the Prior Art It has been known heretofore, to provide a leg pivot which is forward of the center of gravity of a vertical doll's leg, so that when suspended from the pivot with the torso erect the leg would swing forwardly toward a position in which the center of gravity of the leg would lie in the same transverse vertical plane as the pivot.

A great many of the prior walking dolls consisted of intricate walking mechanisms, with a plurality of parts including motors. The complexity of the dolls reflected the expense of manufacture and the ultimate high retail cost. None of the prior devices had a walking mechanism which could effectively simulate a realistic walking motion and still remain within a low price range.

Additionally, even the walking dolls of the lower price range included a plurality of components, usually required a separate pivot pin for the leg pivots, and necessitated considerable costs in assembly.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a walking doll which is so constructed that it is not subject to any of the foregoing disadvantages.

More specifically, it is an object of the present invention to provide a walking doll of the character described which is simple and rugged in construction, yet can be fabricated by mass production methods at an appreciably lower cost than prior devices designed to serve the same function.

It is another object of the present invention to provide a walking doll of the character described which consists of the absolutely minimum number of separate parts, enabling easy and rapid assembly.

It is a further object of the present invention to provide a walking doll of the character described wherein the walking mechanism occupies but a small portion of the space available within the torso.

Yet another object of the invention is to provide a walking doll of the character described wherein a new and improved structure is provided for mounting the legs on the dolls torso.

Still another object of the present invention is to provide a walking doll of the character described wherein improved means is provided for limiting the rotation of the dolls leg during walking.

It is another object of the invention to provide a walking doll of the character described wherein improved means is provided for minimizing lateral rocking of the doll's leg during walking.

Another object of the present invention is to provide a walking doll of the character described wherein the doll walking mechanism is suitable for actuation by manually induced sideto-side rocking of the dolls body as it is moved forwardly.

A further object of the present invention is to provide a walking doll of the character described wherein a simple walking mechanism actuated by side-to-side rocking of the doll torso simulates positive stepwise driving ofthe doll legs.

A further object of the present invention is to provide a walking doll of the character described wherein the doll walking mechanism is suitable for actuation by a self-contained motorized side-to-side rocking means.

Yet another object of the present invention is to provide a walking doll of the character described including improved structure for pivotally mounting the doll's legs on the torso.

Another object of the present invention is to provide a walking doll of the character described wherein a radial projection is integrally formed with each leg for pivoting the leg.

A still further object of the present invention is to provide a walking doll of the character described wherein a means is provided which aids in limiting the respective legs against lateral shifting within the respective leg sockets.

It is another object of the present invention to provide a walking doll of the character described wherein radial projections are provided on leg portions and mating formations are provided within the dolls torso to limit forward and rearward leg motion during simulated walking and to aid in limiting lateral shifting of the legs.

Other objects of the invention in part will be obvious and in part will be pointed out hereinafter.

The invention accordingly consists of the features of construction, combinations of elements and arrangements of parts which will be exemplified in the walking doll hereinafter described and of which the scope of application will be indicated in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS In the accompanying drawings in which are shown various possible embodiments of the invention:

FIG. I is a side view of a walking doll embodying the invention and shown tilted to the right and with the left leg swung forwardly in the position of taking a forward step;

FIG. 2 is an enlarged, fragmentary cross-sectional view taken along the line 2-2 of FIG. 1;

FIG. 3 is an enlarged fragmentary cross-sectional view taken along the line 3-3 of FIG. 2;

FIG. 4 is an enlarged fragmentary cross-sectional view taken along the line 4-4 of FIG. 2 and showing, in dashed lines, the position of the right leg when pivoted forwardly;

FIG. 5 is a view similar to FIG. 4 of an alternate embodiment of the invention, but showing in solid lines the right leg in forward position; and

FIG. 6 is an enlarged perspective view of the upper end of a leg constructed in accordance with the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now in detail to the drawings, the reference numeral 10 denotes a doll having a walking mechanism constructed in accordance with one embodiment of the invention. The doll 10 comprises a hollow torso 12 with a head 14 and arms 16 connected thereto in any conventional manner.

The torso 12 may be constructed of front and back shell 18 and 20, joined along their respective side edges 22 and 24 by any suitable securing means such as a heat seal or an adhesive.

Circular hip openings 26 and 28 are formed at the lower portion of the torso l2 and consist of mating approximately semicircular openings in each shell. The edges of the openings 26 and 28 lie in intersecting planes which are inclined downwardly toward the vertical centerline of the torso 12.

In order to lend structural rigidity for the walking mechanism and support for the dolls legs, the front and rear shells are thickened in the region of the lower torso.

As seen in FIGS. 3 and 4, a horizontal, transverse (side-toside) narrow groove 30 is formed within the thickened lower portion of the front shell 18 at an elevation which is approximately the same as the centers of the circular leg openings 26 and 28. At the bottom of the groove 30 an arcuately curved concave approximately semicircular channel wall 32 is formed and its span begins and terminates entirely within the thickened lower portion. A smooth pivotal bearing surface is formed by the channel wall 32. This bearing surface acts as a bearing for other elements in a manner to be described hereinafter.

Two triangularly shaped flanges 34 and 36 project forwardly toward the center of the hollow torso 12 from the back shell 20. The flanges 34 and 36 are equidistantly'spaced inwardly of the leg openings 26 and 28 (as viewed in FIG. 2) with their opposed parallel sidewalls defining a passageway 38 which is at least downwardly opening. The purpose of said passageway will be described hereinafter. The passageway is centered on the front-to-back vertical central plane of the torso. The lower edges 40 and 42 of the flanges 34 and 36 extend from the back shell upwardly to convexly curved ends which are located above the top elevation of the leg openings. The point of juncture between the edges 40 and 42 and the back wall 20 is below the top elevation of the leg openings 26 and 28.

A U-shaped channel 44 is centrally located on the interior of the back shell 20 (as viewed in FIG. 2) between the leg openings 26 and 28. The channel 44 extends concavely upwardly from the edge 24 at the bottom of the torso back shell 20 to a termination point 46 which is at an elevation that is approximately the same as the centers of the circular leg openings 26 and 28. A portion 45 of the channel 44 which leads to the termination point 46 is arcuate. The center of curvature CC of the arcuate portion 45 and the center of concavity of the transverse channel wall 32 lie in a common side-tosideline.

The doll 10 is supported by the left and right legs 48 and 50 having feet 52 with substantially flat bottoms. At the upper ends 54 and 56 of the legs integrally formed horizontal cylindrical sections 58 and 60 project inwardly toward each other through their respective leg openings 26 and 28 and abut each other at their opposed flat ends 59 and 61. Each of the cylindrical sections 58 and 60 join the respective leg ends 54 and 56 along juncture surfaces 62 and 64 lying within planes which are parallel to the planes ofthe leg openings 26 and 28.

To permit vertical movement of the cylindrical portions 58 and 60 the diameters of the cylindrical portions 58 and 60 are somewhat less than the vertical clearance within the leg openings 26 and 28. Thus the movement ofthe cylindrical sections within the openings 26 and 28 is not obstructed by the leg openings.

Integral ribs 66 and 68 project radially forwardly from the cylindrical sections 58 and 60, respectively. The ribs 66 and 68 have convexly curved outer surfaces which conform to the channel wall 32 to be rotatably received therein. The sides of the groove 30 flare rearwardly to permit rocking of the ribs 66 and 68 within the bottom of said groove.

Integral flat nibs 70 and 72 project radially rearwardly from the cylindrical sections 58 and 60 respectively adjacent the respective ends 59 and 61 and toward the back shell 20. The nibs are diametrically opposite to the ribs 66 and 68 and are conjointly matingly received within the channel 44 (see FIG. 3). Said nibs are free to move within the arcuate channel portion 45 to the termination point 46 thereof when said cylindrical sections are pivoted about the hinge formed by the ribs 66 and 68 within the channel wall 32.

Flat tongues 74 and 76 are integrally formed with the cylindrical sections 58 and 60 respectively and project radially upwardly therefrom adjacent the respective ends 59 and 61. The tongues are conjointly received within the passageway 38.

The radial projections 66 and 68, 70 and 72, 74 and 76 all have one sidewall flush with the respective ends 59 and 61 of the cylindrical sections.

The combined breadth of the ribs 66 and 68 is slightly less than the width of the groove 30. The combined breadth of the nibs 70 and 72 is slightly less than the width of the channel 44. The combined breadth of the tongues 74 and 76 is slightly less than the width of the passageway 38. Thereby transverse shifting or rocking of the legs is minimized after the legs have been incorporated into the torso.

To assemble the walking mechanism, the cylindrical portions 58 and 60 are inserted into the respective leg openings before the front and back shells are secured together. In their assembled positions, the cylinder ends 59 and 61 abut each other as the ribs 66 and 68 are engaged within the horizontal groove 30. Said ribs 66, 68 and groove 30 conjointly constitute a means pivotally suspending the legs 48, 50 about a pivotal axis at the center of curvature CC from the torso for wardly of the center of gravity CGL of said legs. Additionally, the nibs 70 and 72 are positioned within the channel 44 and the tongues 74 and 76 are positioned within the passageway 38.

In the assembled position, with the doll standing erect, the center of gravity CGT of the torso portion 12, with the head and arms attached, lies rearwardly of the horizontal groove 30, and within the lower portion of the torso 12. Thus, when standing erect the doll torso would tend to pivot towards to backward leaning direction about either or both ribs 66 and 68 of the legs 48 and/or 50. The torso does pivot backward a slight distance until the nibs 70 and/or 72 abut the termination point 46 ofthe channel 44.

The doll 10 is placed upon a flat surface in an erect standing position. Both legs are vertical. To actuate the walking mechanism, the child merely grasps the upper portion of the doll 10, by the shoulders or an arm, for example, and tilts the doll sideways without twisting the doll from a forwardly facing direction.

If the doll is tilted to the right (the left as viewed in FIGS. 2 and 3) the right leg 50 remains in contact with the flat surface and the left leg 48 is lifted off said surface and is free to hang. Because the cylindrical section 58 is of a smaller diameter than the vertical clearance of the opening 26, the leg 48 and the cylindrical section 58 would tend to move downwardly and also pivot in a lateral clockwise direction as viewed in FIG. 2 about the bottom of the leg opening 26. The bottom of the cylindrical section is prevented from contacting the bottom of the leg opening 26 and hence the lateral clockwise pivoting of the leg is prevented because the flat end wall 59 of the cylindrical section 58 abuts the flat end wall 61 of the cylindrical section 60 and because the rib 66 is engaged within the horizontal groove 30 and the tongue 74 slidably abuts the inner sidewall of the flange 34.

Since the leg 48 is off the flat surface, it is free to pivot about a substantially horizontal side-to-side axis CC in a forward direction about the rib 66 which is journaled in the horizontal groove 30 (actually slightly off true horizontal due to the side tilting of the doll) and which rib (also the rib 68) together with the groove 30 jointly constitute the pivotal axis for the legs. Because the pivotal axis is forward of the center of gravity of the leg 48, said leg swings forwardly toward an angular orientation wherein the center of gravity lies directly under the pivotal axis. Full forward pivoting to such angular orientation is prevented, however, because the tongue 74 abuts the interior wall of the back shell 20 and provides a positive stop as illustrated in the dashed line view of FIG. 4. This arrangement i.e., stopping the forwardly swinging leg short of such angular orientation, provides a natural walking movement of the leg inasmuch as it ensures brisk action without a noticeable slowing or vacillation at the end of forward movement.

When the doll is then tilted in the opposite direction to the left (to the right as viewed in FIGS. 2 and 5) the forwardly swung left leg 48 touches the flat supporting surface. The doll is moved forwardly and is tilted further to the left causing the right leg 50 to be swung forwardly to a raised position in a manner similar to that previously described with relation to the left leg 48. As the doll moves forwardly the left leg, in contact with the ground, moves rearwardly relative to the torso (as viewed in FIGS. 1 and 3) until the nib 70 contacts the termination point 46 to prevent further pivoting of the left leg in a counterclockwise direction (as viewed in FIG. 4) about the rib 66 and the horizontal groove 30.

The tongues 74 and 76 brushing against one another and against the inner walls of the passageway 38 and the nibs 70 and 72 brushing against the inner walls of the channel 44 cause the legs to be guided for swinging movement in a foreto-rear vertical plane without appreciable lateral rocking.

Thus, by repeated side-to-side tilting and forward movement of the doll body, a child can cause the doll to stride across a flat supporting surface in a simulated walking motion.

In an alternate embodiment as shown in FIG. 5, the leg structure is identical to that used in the preferred embodiment, however, the torso construction is slightly modified. The modified torso 12a is unitarily constructed of a flexible elastomeric synthetic plastic, e.g., polyvinyl chloride, and the channel 44a is arcuately formed in the lower rear portion of the torso 12 with upper and lower termination points 46a and 46b respectively. The termination point 46b acts as an abutment stop limiting clockwise (forward relative to the torso) rotation of the leg (as viewed in FIG. 5) and the termination point 46a acts as an abutment limiting counterclockwise (rearward relative to the torso) rotation of the leg.

To assemble the alternate embodiment, the flexible elastomeric synthetic plastic is bent slightly as the cylindrical leg portions are inserted into their appropriate leg openings and the protrusions are inserted into the groove 30a, the passageway 38a and the channel 44a. After the legs and protrusions are inserted, the plastic returns to its original shape.

It will be readily understood that a unitarily molded elastomeric synthetic plastic torso could be used in the preferred embodiment wherein there is only a single engageable termination point stop 46 in the channel 44. It will also be readily understood that the embodiment of FIG. 5 could be also fabricated with a torso consisting of two shells, such as that of the preferred embodiment.

The torso of the preferred embodiment can be molded of any suitable material such as a relatively stiff synthetic plastic or material of like stiffness or of an elastomeric synthetic plastic.

It may be noted that the plastic of which the torso is molded is reinforced at the lower section to provide support for both the leg walking mechanism and the remaining doll structure.

The legs are fabricated of a suitable synthetic plastic such as a stiff polyvinyl chloride and are thick shells or solid in order to make the same sufficiently heavy to create enough torque when the doll is tilted side to side to overcome the static and kinetic functions at the various nibbing surfaces that would inhibit brisk forward swinging movement of the legs. Moreover, the various rubbing surfaces are made smooth so as to offer low frictional resistence to the pivoting.

Thus it will be seen that there have been provided walking dolls which achieve the various objects of the invention and which are well adapted to meet the conditions of practical use.

As various possible embodiments might be made of the present invention and as various changes might be made in the embodiments above set forth, it is to be understood that all matter herein described or shown in the accompanying drawings is to be interpreted as illustrative and not in a limiting sense.

Having thus described the present invention, I claim as new and desire to be secured by Letters Patent:

1. A walking doll having legs adapted to raise and lower in a stepwise fashion as the doll is moved across a supporting surface and tilted from side to side, said doll comprising a torso, a doll head and doll arms connected to said torso, opposed leg receiving openings located adjacent the lower end of said torso, a pair of legs each having a longitudinal axis and a foot positioned at the bottom of said leg, said legs being swivelably mounted to said torso by mounting means within said torso, each of said legs having a portion extending through and smaller than one of said openings, said leg portion projecting from the upper ends of said legs inwardly toward the interior of said torso, each said portion having an approximately horizontal axis, pivotal means within said torso suspending said legs forwardly of the respective centers of gravity of said legs, said pivotal means comprising a projection extending forwardly from the periphery of each said portion in a direction radially of said horizontal axis, a cooperating mating recess means within said torso whereby each of said legs will pivot about said pivotal means to a forward stepwise position when suspended above said supporting surface, and means within said torso cooperating with said legs for limiting the forward and rearward pivotal movement of said legs with respect to said torso.

2. The walking doll of claim I wherein said means for limiting the pivotal movement of said legs includes for each of said portions a second projection extending radially of the horizontal axis of such portion and rearwardly from the periphery thereof, a cooperating mating first abutment within the said torso, said first abutment and said second projection being spaced from each other an arcuate distance when said leg is in a forward stepwise position and engaging one another when the leg is in its rearward stepwise position, the center of curvature of the arc of said arcuate distance being at said pivotal means, whereby when each leg is pivoted rearwardly said second projection will abut said first abutment after a predetermined degree of rotation from said forward position.

3. The walking doll of claim 2 wherein said means for limiting the pivotal movement of the legs further includes for'each of said portions a second cooperating mating abutment positioned within said torso spaced from said first abutment in a position to contact said second projection when said leg is in its forward stepwise position, said second projection and said second abutment being spaced from each other a second arcuate distance when said leg is in a vertical position, the center of curvature of the arc of said second arcuate distance being at said pivotal means, said second projection abuttingly contacting said second abutment when said leg is suspended above said supporting surface and before said leg achieves a freely balanced fully suspended position whereby the said leg is prevented from oscillating or slowing at the end of a forward step movement.

4. The walking doll of claim 3 wherein said cooperating mating first abutment is positioned at one end of an arcuate channel, the center of curvature of the arc of said arcuate channel being at said pivotal means, said second projections lying within said arcuate channel, said second projections traversing arcuate paths of travel when each of said legs is pivoted from a vertical position to said forward stepwise position, the center of curvature of the arc of said arcuate path being at said pivotal means.

5. The walking doll of claim 2 including for each of said portions, a third projection extending radially of said horizontal axis of such portion and upwardly from the periphery thereof, parallel formations projecting within said torso, said third projection lying substantially between said parallel formations, said third projection traversing an arcuate path of travel when said leg is pivoted from a vertical position to said forward stepwise position, the center of curvature of the arc of said arcuate path being at said pivotal means, said paths of travel being substantially parallel and lying substantially between said parallel formations.

6. The walking doll of claim 5 wherein said means for limiting the pivotal movement of said legs with respect to said body further includes a pivot stop positioned within saidtorso, said pivot stop lying across said paths of travel and being spaced from said third projection a second arcuate distance when said leg is in a vertical position, the center of curvature of the arc of said second arcuate distance being at said pivotal means, said third projections engaging said pivot stop when each of said legs is suspended and before each of said legs achieves a fully balanced freely suspended position, whereby each of said legs is prevented from oscillating or slowing at the end of the forward step movement.

7. A walking doll of claim 6 wherein said pivot stop is positioned between said parallel formations.

8. The walking doll of claim 1 wherein the ends of said portions include a smooth flat wall on each portion, said flat walls being opposingly engaged within said torso.

9. The walking doll of claim 2 wherein said cooperating mating first abutment is positioned at one end of an arcuate channel, the center of curvature of the arc of said arcuate channel being at said pivotal means, said second projections lying within said arcuate channel, said second projections traversing arcuate paths of travel when each of said legs is pivoted from a vertical position to said forward stepwise position, the center of curvature of the arc of said arcuate path being at said pivotal means. 

1. A walking doll having legs adapted to raise and lower in a stepwise fashion as the doll is moved across a supporting surface and tilted from side to side, said doll comprising a torso, a doll head and doll arms connected to said torso, opposed leg receiving openings located adjacent the lower end of said torso, a pair of legs each having a longitudinal axis and a foot positioned at the bottom of said leg, said legs being swivelably mounted to said torso by mounting means within said torso, each of said legs having a portion extending through and smaller than one of said openings, said leg portion projecting from the upper ends of said legs inwardly toward the interior of said torso, each said portion having an approximately horizontal axis, pivotal means within said torso suspending said legs forwardly of the respective centers of gravity of said legs, said pivotal means comprising a projection extending forwardly from the periphery of each said portion in a direction radially of said horizontal axis, a cooperating mating recess means within said torso whereby each of said legs will pivot about said pivotal means to a forward stepwise position when suspended above said supporting surface, and means within said torso cooperating with said legs for limiting the forward and rearward pivotal movement of said legs with respect to said torso.
 2. The walking doll of claim 1 wherein said means for limiting the pivotal movement of said legs includes for each of said portions a second projection extending radially of the horizontal axis of such portion and rearwardly from the periphery thereof, a cooperating mating first abutment within the said torso, said first abutment and said second projection being spaced from each other an arcuate distance when said leg is in a forward stepwise position and engaging one another when the leg is in its rearward stepwise position, the center of curvature of the arc of said arcuate distance being at said pivotal means, whereby when each leg is pivoted rearwardly said second projection will abut said first abutment after a predetermined degree of rotation from said forward position.
 3. The walking doll of claim 2 wherein said means for limiting the pivotal movement of the legs further includes for each of said portions a second cooperating mating abutment positioned within said torso spaced from said first abutment in a position to contact said second projection when said leg is in its forward stepwise position, said second projection and said second abutment being spaced from each other a second arcuate distance when said leg is in a vertical position, the center of curvature of the arc of said second arcuate distance being at said pivotal means, said second projection abuttingly contacting said second abutment when said leg is suspended above said supporting surface and before said leg achieves a freely balanced fully suspended position whereby the said leg is prevented from oscillating or slowing at the end of a forward step movement.
 4. The walking doll of claim 3 wherein said cooperating mating first abutment is positioned at one end of an arcuate channel, the center of curvature of the arc of said arcuate channel being at said pivotal means, said second projections lying within said arcuate channel, said second projections traversing arcuate paths of travel when each of said legs is pivoted from a vertical position to said forward stepwise position, the center of curvature of the arc of said arcuate path being at said pivotal means.
 5. The walking doll of claim 2 including for each of said portions, a third projection extending radially of said horizontal axis of such portion and upwardly from the periphery thereof, parallel formations projecting within said torso, said third projection lying substantially between said parallel formations, said third projection traversing an arcuate path of travel when said leg is pivoted from a vertical position to said forward stepwise position, tHe center of curvature of the arc of said arcuate path being at said pivotal means, said paths of travel being substantially parallel and lying substantially between said parallel formations.
 6. The walking doll of claim 5 wherein said means for limiting the pivotal movement of said legs with respect to said body further includes a pivot stop positioned within said torso, said pivot stop lying across said paths of travel and being spaced from said third projection a second arcuate distance when said leg is in a vertical position, the center of curvature of the arc of said second arcuate distance being at said pivotal means, said third projections engaging said pivot stop when each of said legs is suspended and before each of said legs achieves a fully balanced freely suspended position, whereby each of said legs is prevented from oscillating or slowing at the end of the forward step movement.
 7. A walking doll of claim 6 wherein said pivot stop is positioned between said parallel formations.
 8. The walking doll of claim 1 wherein the ends of said portions include a smooth flat wall on each portion, said flat walls being opposingly engaged within said torso.
 9. The walking doll of claim 2 wherein said cooperating mating first abutment is positioned at one end of an arcuate channel, the center of curvature of the arc of said arcuate channel being at said pivotal means, said second projections lying within said arcuate channel, said second projections traversing arcuate paths of travel when each of said legs is pivoted from a vertical position to said forward stepwise position, the center of curvature of the arc of said arcuate path being at said pivotal means. 